Scientists have found a fossil dating back at least 16 million years of a female shrimp-like creature with enormous fossilized sperm in her reproductive tract. It's a unique example of a female that copulated just before she died and started to turn to stone.
The fossil is a display of "ancient sex with gargantuan sperm," says the lead scientist, Renate Matzke-Karasz of German's Ludwig-Maximilian-University, via e-mail. "We have here direct evidence of a recent mating. … All the co-authors are still amazed by the findings."
The post-coital specimen is an ancient example of a mussel shrimp, technically known as an ostracod. These tiny animals have hinged shells like a mussel's and live today in watery places from flower pots to the ocean, where they subsist on detritus in the water. The fossil specimens were discovered in an Australian cave where large numbers of bats roosted millions of years ago. The bats unwittingly made a major contribution to science: Their guano, Matzke-Karasz says, supplied chemicals that helped preserve the finest details of the mussel shrimps' anatomy.
The scientists found four fossilized female mussel shrimp and one male mussel shrimp with sperm in their bodies, some of the oldest fossilized sperm found to date. When they examined the fossilized male, "we almost couldn't believe our eyes," Matzke-Karasz says. The animal was replete with "sperm (that) looked like little ropes, exactly how modern ostracod giant sperm look!"
The mussel shrimp may be small, but the modern male is mighty, producing so-called "giant sperm" that can be four times longer than the animal itself. Only a handful of other animals, including some flies and moths, make giant sperm, whose purpose is still unclear.
The new study, appearing in this week's Proceedings of the Royal Society B: Biological Sciences, shows that male mussel shrimp may have been deploying giant sperm for more than 140 million years, says micropaleontologist David Horne of Britain's Queen Mary University of London.
"No one knows why ostracods have giant sperm or how they originated, and the new evidence that they have been around for millions of years only adds to the mystery," Horne says via e-mail, calling the fossils "amazing."
Sperm packets from insects have been found in amber from the Early Cretaceous period, and insects frozen in the act of mating are preserved in amber dating back 130 million years, says George Poinar of Oregon State University. Love between turtles has also been immortalized in the form of fossilized male-female pairs of the animals nestling close to each other, some with the males' tails in a telling position.
Mussel shrimp have a wide variety of strange – to us, anyway – sexual practices. Many have two sets of reproductive organs that act in parallel during mating. Some species have three genders: males, females, and females that don't have sex with the males.
"It's really bizarre," Matzke-Karasz says. "We have it all in ostracods."